Id proteins were originally characterized as inhibitors of DNA binding and cell differentiation. Id1 through 4 contain an evolutionarily conserved helix-loop-helix (HLH) sequence which is critical for protein-protein interaction(s). Most HLH transcription factors contain a basic amino acid region adjacent to the HLH sequence, the bHLH sequence, which is responsible for DNA binding. bHLH transcription factors fall into 2 major groups designated class A factors, e.g., E2.2 and E47, and class B factors, e.g., MyoD, myogenin. In vitro studies demonstrate distinct interaction(s) between Id proteins and bHLH transcription factors. While Id proteins contain an HLH domain, they lack the basic region which is required for DNA binding. Therefore, Id proteins are negative regulators of transcription since complexes which contain them do not bind DNA. Id proteins are variably expressed throughout the cell cycle and are regulated by phosphorylation by cyclin-cdk complexes. Thus, Id proteins play an important role in transcriptional regulation of cell cycle related genes. Overexpression of Id1 can induce apoptosis in serum-starved fibroblasts and is correlated with cell cycle progression promoted by Id family members. For example, Id2 can reverse the cell cycle block provided by retinoblastoma protein (Rb) via direct interaction between Id2 and Rb. Clone B31.1 has been reported to recognize mouse Id2, but does not crossreact with mouse Id1 or mouse Id3.